The present invention is related to alternators/electric machines, and is more specifically related to spark protected alternators in which the interface between commutating rings and brushes is effectively isolated from the ambient atmosphere surrounding the alternator.
It is known that some alternators must be operable in potentially explosive ambient atmospheres. However, spark problems exist for alternators which utilize conventional construction in which commutating (slip) rings on an alternator shaft mate with associated brushes so as to apply excitation to a rotor assembly on the shaft. In such conventional alternators sparks may occur due to the electrical and mechanical contacts formed between the brushes and the commutating rings. In prior spark protected alternators, the solution was to provide a total sealed external cavity for enclosing the commutating rings and the entire brush assembly. The brush assembly includes the brushes and the brush holder to which the brushes are attached. The external cavity was adjacent to an end plate of the alternator housing and was typically separate from an interior alternator housing cavity in which the rotor and stator assemblies were located. Generally the voltage regulator for the alternator was also in this sealed external cavity.
The prior art sealed external cavity, besides providing spark protection, also provided dust protection for the brush to the commutating ring connection, as well as for other alternator components. The sealed external cavity was formed by either a solid cup-shaped end cap which was placed over an end plate of the alternator housing, or a flat end cap plate which mated with extending side walls of the alternator housing which formed the side walls of the external cavity. In either case, efficiently sealing this large external cavity proved difficult and expensive since any leakage around the perimeter of this large cavity could result in a catastrophic explosion if the alternator were operated in an explosive ambient atmosphere. In addition, providing extensions of the alternator housing to form the majority of the side walls forming this external cavity, while minimizing some potential leakage problems of the cavity to the ambient atmosphere, greatly increased the weight of the alternator since typically the alternator housing comprises heavy cast metal components.
In addition to the above deficiencies of the prior art, providing a large external cavity adjacent to the alternator end plate for the brush and commutating ring assemblies typically utilized a substantial amount of area of the alternator end plate. Most alternators rely on air cooling of the voltage regulator, the rectifier diodes and the rotor and stator assemblies. Utilizing a large external sealed cavity adjacent to the alternator end plate reduced the cooling efficiency of these alternators since the cavity prevented having extensive air ventilation holes in the end plate, and since the regulator and diodes were typically positioned in the sealed cavity. These factors reduced the power rating of these alternators. Therefore, in order to produce spark protected and nonspark protected alternators having the same maximum power output rating there were extensive differences in alternator designs for spark protected and nonspark protected alternators having the same power rating. This prevented use of a standard design and, therefore, increased alternator cost.